The present invention relates to double-chain stitching. More particularly this invention concerns a method of and apparatus for forming a 401 double-chain stitch.
In a double-chain stitch a needle thread is poked down by a needle through the goods being sewn to form a loop on the underside of the goods. A looper guiding a looper thread engages the loops and locks them with the looper thread, the looper moving transversely to the stitching direction. As the looper engages the needle-thread loops, they slide on it and are pulled into a so-called triangle through which the needle pokes with its next stitch to form another needle-thread loop that locks the previous stitch and that is in turn engaged by the looper.
When seaming with a double-chain stitch in a straight line there is no problem with the looper feeding the looper thread through the needle-thread loops, even at high speeds of as much as 4000 to 6000 stitches per minute. The natural advance of the goods being stitches pulls open the above-mentioned triangle so that the needle can engage accurately through it and form the interlocked double-chain stitch. If the needle does not engage through this filament triangle, stitches will be dropped and the seam will not hold.
Dropping stitches is most likely when the stitch length is very short, as the size of the opening through which the needle must engage is very small. Furthermore when the stitch is a zig-zag or follows a nonstraight path, it is possible to drop stitches at the corners. At the end of the seam, when the stitch direction is typically reversed to lock the seam, this problem is compounded as then the movement of the goods will not pull the threads into a position allowing the needle to engage through them and form the appropriately locked chain stitch.
Accordingly it has been suggested to provide a looper finger that engages the threads below the stitching plane to form a thread triangle. Such an arrangement does not work well when reverse stitching, and the mechanical structure itself is not very reliable. Furthermore the threads are often broken with such a system, particularly when working at high speed. As a result this system produces dropped stitches to such an extent as not generally to be usable.
It is therefore an object of the present invention to provide an improved method of and apparatus for double-chain stitching.
Another object is the provision of such an improved method of and apparatus for double-chain stitching which overcomes the above-given disadvantages, that is which allows non-straight and reverse stitching to be done at high speed without dropping stitches.
A double chain stitch is made has according to the invention by first pushing a needle carrying a needle thread down through goods to be seamed to form below a lower face of the goods a needle-thread loop and then passing a looper carrying a looper thread transversely to a stitching direction parallel to the face through the loop below the lower face to pass the looper thread through the needle-thread loop. Then the looper thread and needle-thread loop are gathered on a catching element below the lower face and the element is moved with the gathered needle and looper threads away from the needle and looper generally parallel to the face while advancing the goods in the direction to form with the needle-thread loop and looper thread a window. Thereafter the needle is again passed through the window and the gathered needle and looper threads are released from the element so that they engage around the most recently formed needle loop. The steps are then repeated.
With this method the triangular window is formed in every case and positioned such that the needle pokes down through it. Thus no stitches are dropped. With normal straight-ahead stitching, the element is not strictly necessary, but when stitching is along a curved path or is reversed, the element is operated to ensure that the window is formed for locking stitches. The element forms the window, so that even if very short stitches are being made or the stitching direction is reversed, the window will be created for the next loop.
The catching element is pulled in the direction in which the goods are advanced. Thus when the goods are reversed at the end of a seam the window will be created ahead, relative to the new travel direction of the goods, of the needle so it will poke down through it as required.
The element is also moved transversely to the direction opposite to the looper. This ensures that the looper thread will not come off the looper, even with a direction change, and that the appropriate window will be formed.
The catching element according to the invention is moved in an annular path synchronously with vertical reciprocation of the needle and transverse displacement of the looper. It can ride in an annular cam groove and be connected by an arm to the needle or looper drive to ensure such synchronous action. Alternately the drive for the element can be a wholly separate servosystem that only operates when needed, that is when the stitching direction changes or is reversed.
The double-chain stitching apparatus in accordance with the invention has according to the invention a needle carrying a needle thread, a needle drive for pushing the needle down through goods to be seamed to form below a lower face of the goods a needle-thread loop, a looper carrying a looper thread, and a looper drive for pushing the looper transversely to a stitching direction parallel to the face through the loop below the lower face to pass the looper thread through the needle-thread loop. An element between the lower face and the looper has a drive for gathering the looper thread and needle-thread loop at the lower face on an element below the lower face, pulling the element with the gathered needle and looper threads away from the needle and looper generally parallel to the face while advancing the goods in the direction to form with the needle-thread loop and looper thread a window, and releasing the gathered needle and looper threads from the element so that they engage around the needle loop.
The element moves in an annular path in a plane parallel to the goods and has a pair of generally parallel engagement surfaces meeting at a corner.